The mineral arsenopyrite, in some instances, is known to contain gold and silver which are in solution in the mineral matrix or are present as fine inclusions in the mineral. The gold and silver are not available for extraction by conventional hydrometallurgical processes such as cyanidation which treat only the mineral surfaces. The mineral pyrite is often associated with arsenopyrite and these minerals may contain in their matrices finely dispersed gold which is difficult to extract.
The conventional means of liberating gold from pyrite and arsenopyrite concentrates is to roast the material and then treat the calcine by cyanidation. This process generates environmental pollution problems due to the airborne emission of sulphur and arsenic oxides. The tailings from the calcine cyanidation contain arsenic which is also a potential environmental contaminant.
Arsenopyrite and pyrite concentrates may also be treated for gold recovery through conventional pyrometallurgical processes which include copper smelting, lead smelting and zinc roasting. These processes also produce potentially harmful airborne arsenic emissions from the treatment of these concentrates. Problems associated with the added arsenic burden in the process flows also arise.
Two hydrometallurgical processes exist which could potentially be used to decompose arsenopyrite and pyrite concentrates though they are not specifically used for this purpose. These are the Sill and the Calera processes which are both used for the treatment of cobalt and arsenic-bearing materials. In the Sill process, the concentrate is solubilized by the action of a caustic substance and oxygen under elevated temperatures and pressures. In the Calera process, sulphuric acid and oxygen at high temperature and pressure are the active agents. Neither process, as far as is known, is commercially operated at the present time.
U.S. Pat. No. 3,793,429, Queneau, February, 1974, discloses a process for treating chalcopyrite and pyrite concentrates in an aqueous slurry for copper recovery while at the same time rejecting iron (column 1, lines 71-72, column 2, lines 1-5). Technology relating to the production of a copper-enriched solution from such chalcopyrite concentrates for the purpose of recovery of copper from the solution, while at the same time rejecting iron and sulphur to the leach residue, is not of much assistance in dealing with the objective of producing a pyrite or arsenopyrite leach residue suitable for gold recovery, while maintaining silver in the liquid fraction.
Queneau conducts his decomposition leach by continuously adding nitric acid to the aqueous slurry in quantities sufficient to completely decompose the chalcopyrite and pyrite concentrates. Queneau continuously removes the nitric oxide resulting from the decomposition reaction and externally generates nitrogen dioxide by the addition of oxygen. The nitrogen dioxide is then absorbed in water to form nitric acid which is recycled to the process. Queneau's process is very slow, particularly in decomposing pyrite, because the nitric acid regeneration step is extremely slow. Also, the nitric acid leaching is very slow.
The Queneau process purports to achieve 98 percent recovery of copper from the solution and gold recovery of 80 percent and silver of 10 percent from the residue (column 4, lines 53-57). Such a low gold recovery from the residue may be acceptable where the gold represents only a by-product from a copper solution recovery process, but it is not acceptable when the principal objective is to treat gold-bearing arsenopyrite and pyrite concentrates. Gold recovery by traditional roasting and cyanidation of such concentrates is generally from 90 percent to 95 percent.
One of the objectives of the Queneau process is to precipitate iron from the solution to produce a purified copper solution. This precipitation is done by removing the nitric oxide and thereby reducing the acidity of the solution. Lowering the acidity of the solution promotes basic iron sulphate precipitation.
It is well known in the art that when iron is precipitated as basic iron sulphate, any silver present in the solution is chemically bonded to and precipitates with the basic iron sulphate. It is then not economically feasible to recover the silver from the basic iron sulphate precipitate. Since the Queneau process does not achieve gold recovery levels of at least 90 percent, and silver is lost with the basic iron sulphate precipitate, Queneau's process is not suitable for the recovery of gold and silver from arsenopyrite and pyrite concentrates and ores.
The Queneau process also has a number of other serious shortcomings. In order to achieve the extraction level indicated in the Queneau patent, several steps must be followed. The concentrate must be ground very fine, for example, minus 270 mesh (53 microns) to minimize retention times. The leaching time is lengthy and multistaged: one hour for acid addition and two hours for nitrate reduction. The nitric oxide gas that is produced is oxidized separate from the leach vessel with the attendant need for gas-handling facilities. Unleached concentrate must be recovered by flotation of the leach residue and then recycled to the leach. Prior to the flotation of unreacted sulphides, the sulphur must be removed from the residue.
U.S. Pat. No. 4,331,469, W. Kunda, May 25, 1982, discloses a process for recovering silver from silver bearing concentrates which in some cases also contain iron and arsenic. Kunda teaches the use of a nitric acid system together with the use of a chloride salt for silver precipitation and pH increase to between 0.8 to 1.8 for iron precipitation. The use of a chloride salt makes it impossible to recycle the process solution in a gold recovery process as it would solubilize gold in the leach stage. The pH increase process for iron rejection yields a precipitate which is chemically unstable with respect to arsenic redissolution and has poor handling characteristics.